Is it possible doctor could of missed twin at 6 week ultrasound?

I know this is going to sound crazy but I had an ultrasound at 6 weeks 1 day which found 1 baby with a healthy heartbeat. I'm now 11 weeks and wondering if there's anyway I could be pregnant with twins? I know that's highely unlikely. Wirh my first pregnancy (son is currently 11 months) I had a very easy pregnancy with hardly any symptoms at all. This pregnancy I have had a lot of dizziness and low blood pressure along with morning sickness and food aversions and my stomach popped so much earlier than it did with my son. I know every pregnancy is different and that's probably the case here but just wondering if anyone has had twins and they were missed on a 6 week early ultrasound?